Autotaxin (ATX; also known as ENPP2 (ectonucleotide pyrophosphatase/phosphodiesterase 2) or lysophospholipase D) is a ˜120 kDa protein that belongs to the ENPP family of enzymes which is composed of seven members, out of which ENPP1 and ENPP3 are the closest to ATX. Whereas ENPP1 and ENPP3 are active in converting ATP into pyrophosphate (a regulator of mineralization and calcification processes in bone), ATX is the only ENPP enzyme with lysophospholipase D (lysoPLD) activity and is responsible for the hydrolysis of lysophosphatidylcholine (LPC) to produce the bioactive lipid lysophosphatidic acid (LPA). Several pieces of evidence have established ATX as the main source of LPA in blood. For example, blood LPA and ATX levels have been shown to be strongly correlated in humans. In addition, LPA levels are reduced by 50% in mice carrying a heterozygous null mutation of ATX (Tanaka, et al., 2006).
Due to the importance of LPA as a biological mediator, the levels of bio-active LPA are expected to be strictly spatially and temporally controlled. The relatively short half life of circulating LPA (˜3 min) in mice is in line with this expectation. In the circulation, where LPC levels are very high (100-200 μM, mainly albumin-bound), ATX is constitutively active but newly produced LPA is rapidly degraded by membrane-bound phosphatases and levels of plasma LPA are thereby kept low (in the low μM range). This is confirmed by the fact that in cell-free plasma ex vivo, LPA levels increase at a steady rate. In addition, LPA in blood is bound to serum albumin, which might further reduce the levels of bio-active LPA. Besides this first level of control of LPA levels, the spatial control of LPA production is ensured by the capacity of ATX to bind to cell surface molecules such as integrins and heparan sulphate proteoglycans (HSPs) to facilitate LPA release near to its cognate receptors. Several pieces of evidence support this hypothesis. First, the structural studies of ATX are supporting the fact that the ATX structure is compatible with such a process (Hausmann, J, 2011). In addition, several reports indicated how ATX is involved in lymphocyte homing through the interaction with cell surface integrins (Kanda, 2008). It was shown, for example, that ATX can be induced on high endothelial venules (HEVs) on sites of inflammation. This ATX expressed by HEVs acts on HEVs in situ to facilitate lymphocyte binding to endothelial cells (Nakasaki, et al., 2008). As such, ATX not only drives the formation of LPA but, through these cellular interactions, also ensures specificity in LPA signaling.
ATX is widely expressed, with highest mRNA levels detected in brain, lymph nodes, kidney, and testis. Originally discovered as ‘autocrine motility factor’ in melanoma cells, ATX has emerged as the key LPA-producing enzyme in plasma and tissues. Unfortunately, embryonic lethality has hampered studies of the importance of ATX in adult life. This embryonic lethality reflects the key role of LPA in various developmental processes, vasculogenesis in particular. Knock-out studies of the LPA receptors have been more informative in terms of unraveling the physiological role of LPA. LPA acts through at least six distinct (G protein)-coupled receptors (LPA1-6) found on the surface of different cell types, three of which belong to the edg receptor family and three to the non-edg receptor family. LPA interacts with specific G protein-coupled receptors (GPCRs), namely LPA1 (also known as EDG2), LPA2 (also known as EDG4), LPA3 (also known as EDG7), LPA4 (also known as GPR23/p2y9), LPA5 (also known as GPR92/93), LPA6 (also known as p2y5). LPA has also been described to interact with three other GPCRs (GPR87, p2y10, GPR35). In addition, a preference of LPA receptors for specific LPA species has been demonstrated (Bandoh, et al., 2000). As such, the specificity of the LPA activities is controlled by the expression pattern of the LPA receptors and their downstream signaling route.
The main part of the LPA responses are mediated through trimeric G-proteins and include but are not limited to mitogen-activated protein kinase (MAPK) activation, adenylyl cyclase (AC) inhibition/activation, phospholipase C (PLC) activation/Ca2+ mobilization, arachidonic acid release, Akt/PKB activation, and the activation of small GTPases, Rho, ROCK, Rac, and Ras. Other pathways that are affected by LPA receptor activation include cell division cycle 42/GTP-binding protein (Cdc42), proto-oncogene serine/threonine-protein kinase Raf (c-RAF), proto-oncogene tyrosine-protein kinase Src (c-src), extracellular signal-regulated kinase (ERK), focal adhesion kinase (FAK), guanine nucleotide exchange factor (GEF), glycogen synthase kinase 3b (GSK3b), c-jun amino-terminal kinase (JNK), MEK, myosin light chain II (MLC II), nuclear factor kB (NF-kB), N-methyl-D-aspartate (NMDA) receptor activation, phosphatidylinositol3-kinase (PI3K), protein kinase A (PKA), protein kinase C (PKC), and ras-related C3 botulinum toxin substrate 1 (RAC1). The actual pathway is influenced by cell type, expression level of a receptor or signaling protein, receptor usage, and LPA concentration (Tania, Khan, Zhang, Li, & Song, 2010). LPA has a broad range of physiological actions and various cellular effects (for example blood pressure regulation, platelet activation, smooth muscle contraction, cell growth, cell rounding, neurite retraction, actin stress fiber formation and cell migration). In addition, a preference of LPA receptors for specific LPA species has been demonstrated (Bandoh, et al., 2000). The knock-out studies for these receptors indicated a role in bone development (Gennero, et al., 2011), and neurogenesis (Matas-Rico, et al., 2008), embryo implantation (Ye, et al., 2005) and the formation of blood and lymphatic vessels (Sumida, et al., 2010).
With regard to pathophysiology, a role for LPA and LPA receptors has been claimed in various patho-physiological conditions such as proliferative diseases, neuropathic pain, inflammation, autoimmune diseases, fibrosis, lymphocyte tracking in lymph nodes, obesity, diabetes, or embryonic blood vessel formation.
The role of LPA in lung fibrosis has been well described in literature and also an involvement in asthma has been claimed. The present inventors however are the first to report a link to chronic obstructive pulmonary disease (COPD).
Several lines of evidence suggest a role for ATX in the control of lung function in disease through effects on lung epithelial cells, fibroblasts and smooth muscle cells. In general, inflammatory conditions in the lung are often described as associated with increased ATX and LPA levels. Instillation of LPS in mice, for example, induces increased ATX and LPA levels in the broncho-alveolar lavage (BAL) fluid (Zhao, et al., 2011). Also in humans, a segmental LPS challenge led to increased LPA levels (Georas, et al., 2007). Overall, the role of LPA in activating lung epithelial cells, the first line of defense to inhaled noxious stimuli, towards increased cytokine and chemokine production and migration have been extensively described (Zhao & Natarajan, 2013). Exogenous LPA promotes inflammatory responses by regulating the expression of chemokines, cytokines, and cytokine receptors in lung epithelial cells. In addition to the modulation of inflammatory responses, LPA regulates cytoskeleton rearrangement and confers protection against lung injury by enhancing lung epithelial cell barrier integrity and remodeling.
In the asthmatic individual, the release of normal repair mediators, including LPA, is exaggerated or the actions of the repair mediators are inappropriately prolonged leading to inappropriate airway remodeling. Major structural features of the remodeled airway observed in asthma include a thickened lamina reticularis (the basement membrane-like structure just beneath the airway epithelial cells), increased numbers and activation of myofibroblasts, thickening of the smooth muscle layer, increased numbers of mucus glands and mucus secretions, and alterations in the connective tissue and capillary bed throughout the airway wall. ATX and/or LPA may contribute to these structural changes in the airway, for example ATX and/or LPA are involved in acute airway hyperresponsiveness in asthma. The lumen of the remodeled asthmatic airway is narrower due to the thickening of the airway wall, thus decreasing airflow. Additionally, LPA contributes to the long-term structural remodeling and the acute hyperresponsiveness of the asthmatic airway, for example LPA contributes to the hyper-responsiveness that is a primary feature of acute exacerbations of asthma. Reports describing the role of LPA in asthma generated different conclusions, ranging from a protective role (Zhao, et al., 2009) to a negative role (Emo, et al., 2012). The testing of autotaxin inhibitors in models for airway diseases as described herein allows for the clarification of the potential of this enzyme as a drug target.
Fibroblast proliferation and contraction and extracellular matrix secretion stimulated by LPA contributes to the fibroproliferative features of other airway diseases, such as the peribronchiolar fibrosis present in chronic bronchitis, and interstitial lung diseases and severe asthma. LPA plays a role in the fibrotic interstitial lung diseases and obliterative bronchiolitis, where both collagen and myofibroblasts are increased. Studies related to IPF (idiopathic pulmonary fibrosis) indicated an increase in LPA levels in the BAL fluid of patients (Tager, et al., 2008). Further LPA1 knock-out and inhibitor studies revealed a key role for LPA in fibrotic processes in lung and were complemented by studies using cell-specific knock-out mice lacking ATX in bronchial epithelial cells and macrophages. These mice were shown to be less sensitive to models of lung fibrosis (Oikonomou, et al., 2012). A role for LPA in other fibrotic diseases (kidney and skin) was based on similar types of observations (Pradere, et al., 2007), (Castelino, et al., 2011). The role of LPA in lung remodeling relates to the effects of LPA on both lung fibroblasts (through LPA1) and epithelial cells (through LPA2) (Xu, et al., 2009) have demonstrated that LPA2 plays a key role in the activation of TGFβ in epithelial cells under fibrotic conditions. The role of LPA in remodeling and fibrosis is relevant to COPD, IPF and asthma, diseases in which lung remodeling as a long term outcome will limit lung function. Finally, of interest towards lung diseases, in mice, ATX is one of the three main QTLs that appear to be associated with differences in lung function (Ganguly, et al., 2007).
One prominent area of research interest is the role of ATX-LPA signaling in cancer (Braddock, 2010). Although cancer-specific mutations in ATX have not been identified so far, overexpression of ATX or individual LPA receptors in xenografted and transgenic mice promotes tumour formation, angiogenesis and metastasis. Conversely, ATX knockdown in mammary carcinoma cells reduces their metastatic spread to bone. Several human cancers show elevated ATX and/or aberrant LPA receptor expression patterns, as revealed by microarray analyses. Autotaxin is viewed as a pro-metastatic enzyme. It has initially been isolated from the conditioned medium of human melanoma cells that stimulates a myriad of biological activities, including angiogenesis and the promotion of cell growth, migration, survival, and differentiation through the production of LPA (Lin M. E., 2010). LPA contributes to tumorigenesis by increasing motility and invasiveness of cells. The initiation, progression and metastasis of cancer involve several concurrent and sequential processes including cell proliferation and growth, survival and anti-apoptosis, migration of cells, penetration of foreign cells into defined tissues and/or organs, and promotion of angiogenesis.
Therefore, the control of each of these processes by LPA signaling in physiological and pathophysiological conditions underscores the potential therapeutic usefulness of modulating LPA signaling pathways for the treatment of cancer. In particular, LPA has been implicated in the initiation or progression of ovarian cancer, prostate cancer, breast cancer, melanoma, head and neck cancer, bowel cancer (colorectal cancer), thyroid cancer, glioblastoma, follicular lymphoma and other cancers (Gardell, 2006) (Murph, Nguyen, Radhakrishna, & Mills, 2008) (Kishi, 2006).
Furthermore, autotaxin is implicated in the invasive and metastatic process of tumor cells, since ectopic overexpression of autotaxin is frequently observed in malignant tumor tissues such as ovarian cancer (Vidot, et al., 2010), breast cancer (Panupinthu, Lee, & Mills, 2010) (Zhang, et al., 2009), prostate cancer (Nouh, et al., 2009), renal cancer, Hodgkin lymphoma (Baumforth, 2005), hepatocellular carcinoma (Wu, et al., 2010), lung cancer (Xu & Prestwich, 2010), and glioblastoma (Kishi, 2006). Autotaxin overexpression has also been found in a variety of tumors such as malignant melanoma, teratocarcinoma, neuroblastoma, non-small-cell lung cancer, renal cell carcinoma (Stassar, et al., 2001).
Furthermore, expression of autotaxin by cancer cells controls osteolytic bone metastasis formation. In particular, LPA stimulates directly cancer growth and metastasis, and osteoclast differentiation. Therefore, targeting the autotaxin/LPA signaling route has also been found to be beneficial in patients with bone metastases (David, 2010). Finally, the inhibition of autotaxin seems to provide a beneficial adjuvant to chemotherapy for preventing tumor growth and metastasis in patients with high autotaxin expression in their tumors (Gaetano, 2009).
Upregulation of the autotaxin-LPA signaling pathway has been observed in a variety of inflammatory conditions. For example, pro-inflammatory effects of LPA include degranulation of mast cells, contraction of smooth-muscle cells and release of cytokines from dendritic cells. As an indication for its general role in inflammation, LPA and autotaxin activity have been shown to be induced by carageenan injection into the mouse air pouch model, which is used to develop anti-inflammatory drugs, including cyclooxygenase inhibitors for arthritis. Furthermore, a reduction in plasma and air pouch LPA has been observed in this rat air pouch model using an autotaxin inhibitor, confirming the role of autotaxin during inflammation as a major source of LPA (Gierse, 2010). It has been observed that autotaxin inhibitors reduce LPA and PGE2 and also reduce inflammatory pain.
As another general role in inflammatory diseases, LPA has been shown to induce chemokinesis in T-cells. Intravenous injection of enzymatically inactive autotaxin has been shown to attenuate the homing of T-cells to lymphoid tissues, likely by competing with endogenous autotaxin and exerting a dominant-negative effect. In certain instances, autotaxin facilitates lymphocyte entry into lymphoid organs (Kanda, 2008). Therefore an autotaxin inhibitor may block lymphocyte migration into secondary lymphoid organs and be of benefit in autoimmune diseases.
Specifically in rheumatoid arthritis, an increased expression of ATX in synovial fibroblasts from RA patients was demonstrated and ablation of ATX expression in mesenchymal cells (including synovial fibroblasts) resulted in attenuated symptoms in mouse models for rheumatoid arthritis (Nikitopoulou, et al., 2012). As such, the role of autotaxin in rheumatoid arthritis has been strongly established.
Several lines of evidence suggest a role for LPA in vascular injury and atherosclerosis. These relate to the role of LPA in modulating endothelial barrier function and the phenotype of vascular smooth muscle cells and the action of LPA as a weak platelet agonist. Platelets have been identified as important participants in LPA production in the circulation in some settings, mainly by providing sufficient LPC amounts. Plasma autotaxin associates with platelets during aggregation and concentrates in arterial thrombus, and activated but not resting platelets bind recombinant autotaxin in an integrin-dependent manner. Experimental induction of thrombocytopenia in rats, using an anti-platelet antibody, decreases the production of LPA in serum by almost 50%, which suggests a role for LPA during clotting. In some instances, transgenic overexpression of autotaxin elevates circulating LPA levels and induces a bleeding diathesis and attenuation of thrombosis in mice. Intravascular administration of exogenous LPA recapitulates the prolonged bleeding time observed in autotaxin-Tg mice. Finally, autotaxin+/− mice, which have ˜50% normal plasma LPA levels, are more prone to thrombosis.
In addition to a role in blood clotting, LPA has multiple effects on the endothelial monolayer permeability increase, and endothelial cells, in particular in critical aspects of angiogenesis such as cell migration stimulation and invasion. Furthermore, LPA also exerts migratory and contractile effects on vascular smooth muscle cells: autotaxin-mediated LPA production and subsequent LPA signaling contributes to vascular development by stimulating endothelial cell migration and invasion as well as regulating adhesive interactions with the extracellular matrix and smooth muscle cells. For example, similar vascular defects have been observed in autotaxin-deficient mice and in mice lacking genes involved in cell migration and adhesion (Van Meeteren, et al., 2006). Therefore an autotaxin inhibitor may have benefit in some diseases involving dysregulated angiogenesis.
LPA induces neuropathic pain as well as demyelination and pain-related protein expression changes via LPA1 (Inoue, et al., 2008). ATX heterozygous knockout mice show about 50% recovery of nerve injury-induced neuropathic pain compared to wild type mice. Lysophosphatidylcholine (LPC), also known as lyso-lecithin, is known to induce neuropathic pain. It has been observed that LPC-induced neuropathic pain is partially reduced in ATX heterozygous knockout mice. These results support the idea that LPA is produced by autotaxin resulting in neuropathic pain (Lin M. E., 2010).
Autotaxin is also implicated in metabolic diseases, in particular obesity and diabetes (Federico, et al., 2012). In some instances, autotaxin is responsible for the lysoPLD activity released by adipocytes and exerts a paracrine control on preadipocyte growth via an LPA-dependent mechanism. In addition, autotaxin is upregulated during adipocyte differentiation and in genetic obesity. In certain instances, autotaxin mRNA is upregulated in adipocytes from db/db mice suggesting that the upregulation of autotaxin is related to the severe type 2 diabetes phenotype and adipocyte insuline resistance. In some instances, upregulation of adipocyte autotaxin is associated with type 2 diabetes in human (Ferry, 2003). The relationship between adipocyte and autotaxin biology suggests the use of an autotaxin inhibitor for the treatment of metabolic diseases.
Finally, two other conditions clearly related to autotaxin biology are cholestatic pruritus (Kremer, et al., 2010) and regulation of ocular pressure (Iyer, et al., 2012).
The current therapies are not satisfactory and therefore there remains a need to identify further compounds that may be of use in the treatment of fibrotic diseases, proliferative diseases, inflammatory diseases, autoimmune diseases, respiratory diseases, cardiovascular diseases, neurodegenerative diseases, dermatological disorders, and/or abnormal angiogenesis associated diseases. The present invention therefore provides compounds, methods for their manufacture and pharmaceutical compositions comprising a compound of the invention together with a suitable pharmaceutical carrier. The present invention also provides for the use of a compound of the invention in the preparation of a medicament for the treatment of fibrotic diseases, proliferative diseases, inflammatory diseases, autoimmune diseases, respiratory diseases, cardiovascular diseases, neurodegenerative diseases, dermatological disorders, and/or abnormal angiogenesis associated diseases.